Facebook passes the following parameters to your application when a user interacts with your application:

* fb_sig_added: If set to true, then the user has authorized your application.

* fb_sig_api_key: Your application's API key.

* fb_sig_friends: The UIDs of the visiting user's friends.

* fb_sig_locale: The user's locale.

* fb_sig_time: The current time, which is a UNIX timestamp.

* fb_sig_user/fb_sig_canvas_user: The visiting user's ID. fb_sig_canvas_user is passed if user has not authorized your application, while fb_sig_user is passed if the user has authorized your application. Neither is passed when an application tab is being requested; fb_sig_profile_user is passed instead.